1. Field of The Invention
The present invention relates to electro-optical transducers and more particularly to an electro-optical tachometer which is adapted to monitor a movable member and outputs electrical signals indicating the speed at which the member is moving, the direction of motion, and other functions relative to the moving member.
2. Description of The Prior Art
The use of an electro-optical tachometer for controlling the velocity of a movable member (e.g. rotating shaft, etc.) via a servo controlled loop, for detecting and measuring direction of motion and for performing other related functions (for example, monitoring movement and output a signal for each increment of motion) are well known in the prior art.
Prior art electro-optical tachometer consists of a fixed assembly, a movable assembly and a light emitting/light receiving assembly. In assemblying the electro-optical tachometer to perform any or all of the previously described functions, the movable assembly is fastened securely to the member whose motion, speed, etc. is to be monitored. For example, if the member is the rotating shaft of a motor then the movable assembly is attached to the rotating shaft. If the member is the head arm of a linear actuator then the movable assembly is fastened to the head arm. Generally, the movable assembly consists of a hub portion suitable for mounting on the movable member. An optical disk, having a plurality of alternating dark and light lines, (sometimes called an optical track), is concentrically mounted to the hub portion. As the shaft rotates, for example, the rotary motion is imparted to the hub and disk. Stated another way, the hub and disk rotate with the shaft.
The fixed assembly consists of a mask having an optical track thereon. The optical track on the mask is similar to the optical track on the rotating disk. The mask is positioned in spaced relationship to the rotating disk. The mask is generally mounted to a fixed portion of the device whose motion is measured. For example, to the housing of a motor or to the frame of a linear actuator. The spacing or gap between the fixed mask and the rotating disk is set by an operator using a shim. The thickness of the shim is equivalent to the spacing which is desired between the fixed mask and the rotating disk.
The light emitting/light receiving assembly is fixed, i.e., immovable and consists of a light emitting source, e.g., light emitting diodes (LED) and a light receiving source, e.g., light receiving transistor (LRT). The LED and the LRT are so arranged that the rotating disk passes therebetween. Thus, as the shaft is rotated the light emitted from the light source is received by the light receiving source each time a light line is positioned within the gap. The change in conductivity of the light receiving source caused by the received light is sensed and by means of appropriate circuitry (e.g., squaring circuitry) a voltage signal is generated. A more detailed description of the prior art electro-optical tachometer is described in U.S. Pat. Nos. 3,693,023 and 3,770,971.
Although the prior art electro-optical tachometer functions satisfactorily for its intended purpose, it is plagued with several problems. The invention disclosed hereinafter is intended to eliminate the prior art problems.
As was mentioned previously in the prior art, the space or gap between the rotating disk and stationary mask is set by an operator using a shim. As is used herein the term shim means a flat thin piece of material having a thickness equivalent to the desired spacing. In actuality, as the tachometer is assembled the operator inserts the shim between the mask and disk. The operator then pulls the shim out and the process is continued until the operator experiences resistance from the mask and disk. At this point it is assumed that the gap is at least equivalent to the thickness of the shim. The operator then tightens the assembly and the gap setting is completed.
Due to the human factor variance there is no repeatability in gap setting. Stated another way, the gap setting is not constant between two electro-optical tachometers even if the spacing is set by the same operator. As is well known to those skilled in the art, the output signals from an electro-optical tachometer are a function of the disk/mask spacing. If the spacing is not constant or if the spacing is too wide, then the ability of the circuits for processing the output from the light receiving source are impaired and the reliability of the electro-optical tachometer to measure a specific function is significantly reduced. In other words, the electro-optical tachometer is unreliable as a measuring instrument.
Still another problem associated with the prior art electro-optical tachometer is that a constant spacing cannot be maintained between the mask and the optical disk. If the spacing changes from an initial setting, for example increases, then the result is the previously described situation. This condition, although expensive and time consuming, may be corrected by resetting the gap.
A more disasterous condition occurs when the spacing narrows to the point where the disk and mask coincide or crash. Disk/mask crash is a result of the disk hub assembly walking on its movable member. For example, in the case of a rotating shaft the disk hub assembly moves on the shaft. Whenever this condition (i.e., disk/mask crash) occurs, the optical pattern (i.e., the track) which is fabricated on the disk and/or mask is obliterated and the tachometer can no longer perform its function.